1. Technical Field
The present invention relates generally to a process for combusting a fuel gas to drive a gas turbine and, more particularly, to a process for combusting a low heating value fuel gas in a combustor by dividing the fuel gas into a plurality of portions and combusting each portion of the fuel gas in a different stage of the combustor.
2. Background Information
Standard commercial combustors for gas turbines employ burner assemblies and combustion chambers typically designed to combust relatively high heating value fuels, such as natural gas which has a heating value in a range from about 50,000 to 60,000 kJ/kg. The stoichiometric demand for air in a conventional commercial burner assembly combusting a high heating value fuel gas is usually at least 10 volumes of combustion air for each volume of fuel gas fed to the burner assembly. In accordance with the swirl-stabilized combustion mechanism utilized by most commercial burner assemblies, the combustion air is fed to the combustion chamber through an annular flow region of the burner assembly which encircles a central burner nozzle. Fixed vanes are positioned in the annular flow region which impart a spin to the combustion air feed creating an air vortex. The burner fuel gas is fed into the interior zone or "eye" of the air vortex via the burner nozzle. The velocities of the feed streams are relatively low at the interface of the fuel-rich edge and the air-rich "eye wall" of the vortex which enables a stable film of laminar diffusion combustion to occur at the interface.
In contrast, the stoichiometric demand for air when combusting a low heating value fuel gas in a burner assembly is substantially less than when combusting a high heating value fuel gas. Low heating value fuel gases contain significant quantities of inert components termed "ballast" gases, such as nitrogen, carbon dioxide, water vapor and the like, which are not combustible and which dilute the remaining combustible components of the fuel gas. Accordingly, the stoichiometric ratio of air to fuel gas in a burner assembly receiving a low heating value fuel gas is generally much less than 10:1 on a volumetric basis, and may be even less than 1:1 for certain fuel gases. As such, a conventional central burner nozzle, which is designed for high heating value fuel gases, is not sufficiently large to accommodate the large volume of low heating value fuel gas required to achieve the necessary heat release for the burner assembly. Conversely, the annular flow region is overly large, permitting an excessive air to fuel ratio in the burner assembly which is outside the flammability envelope of the air/fuel mixture.
Specialized combustors for combusting low heating value fuel gases have been developed at high cost for specific applications, such as for combusting blast furnace gases produced in the manufacture of steel. For example, Asai Brown Boveri Corp. has designed a gas turbine, Model No. 11N-2, which utilizes an oversized burner assembly to accommodate the mass flow of the low heating value fuel gas and utilizes undersized air "swirlers" to ensure a correct ratio of air to fuel in the burner assembly. However, this design is only operable within a narrow range of conditions and cannot be utilized with turbines having internal combustors. Thus, it is apparent that a need exists for an effective process which combusts a low heating value fuel gas and utilizes the resulting gaseous combustion products to drive a gas turbine.
Accordingly, it is an object of the present invention to provide an effective process for combusting a low heating value fuel gas in a combustor associated with a gas turbine and driving the gas turbine with the resulting gaseous combustion products. More particularly, it is an object of the present invention to provide substantially complete combustion of the low heating value fuel gas by dividing the low heating value fuel gas and combusting the divided portions of the low heating value fuel gas in different stages of a combustor. It is another object of the present invention to provide such a combustion process utilizing conventional combustors which are retrofitted at low cost to accommodate the low heating value fuel gas. It is yet another object of the present invention to provide such a combustion process which emits substantially reduced levels of contaminants to the environment. These objects and others are achieved in accordance with the invention described hereafter.